During the last few decades, there has been a lot of research centered along the development of light emitting diodes using inorganic semiconductors for display as well as lighting applications. These efforts could not produce very large and flat luminescent displays which could replace the liquid crystal displays. The reason was that most of the light emitting devices were based on inorganic single crystals and the development of larger single crystals for large area applications were uneconomical. Inorganic thin film displays which were developed in its place also were not efficient enough for commercial applications. Therefore new materials have been sought for light emitting applications. Organic molecular materials have been well known for long time as very efficient fluorescent materials and therefore the attention diverted in this direction. This search has resulted in various electro luminescent organic materials. In 1987, C. W. Tang and Van Slyke, Applied Physics Letters, 51 (1987) 913 have obtained electro luminescence from aluminum tris (8-hydroxy quinoline), the efficiency of which was sufficient to consider commercial exploitation. This finding has created a lot of research interest in the field of organic luminescent materials. New organic electro luminescent (EL) materials have been invented which now cover the full range of the visible spectrum. Apart from small molecules, a number of polymeric materials have also been investigated. Several new polymeric materials have subsequently been found to have considerable electro luminescence efficiency suitable for commercial applications. Even though the EL devices made of molecular materials behave very similar to conventional LEDs made of inorganic semiconductors, they differ in many ways in their electronic and optical characteristics. This is evident from the fact that all molecular materials useful for EL purposes are insulators with resistivities of the order of 1015 to 1020 ohms cm. No electronic charge is present in the EL device without charge injection.
Therefore, charges injected from the electrodes play a crucial role in the device operation and all charges behave as space charge within the device. Further, only neutral excited states are produced by charge recombination and emission is due to emissive transitions from neutral excited states to ground states.
Emission due to direct recombination of positive and negative charges was not observed in molecular materials. An example of such a device using molecular materials is the fabrication of organic LEOs is in using metal chelates and dyes which show very good photoluminescence and electro luminescence (W. Brutting, Stefan Berleb, AG. Muckl, Organic Electronics, 2 (2001) 1–36). Herein, organic layers consisting of two layers namely N,N′-Bis(3-methyl phenyl) N,N′diphenylbenzidine (TPD) and aluminum quinolate (Alq3) are used as hole transport layer and emitter layer respectively which are sandwiched between the anode, Indium tin oxide (ITO) and cathode (Aluminum). Both small molecules based as well as polymer based organic LEDs work on nearly same principles and have nearly the same structure.
Recently much interest has been shown in the use of conductive conjugated polymers for organic light emitting diodes displays. The first scientific literature in the polymer semiconductors diode was published by J. H. Burroughs in Nature 347 (1990) 539 and relates to the use of poly phenylene vinylene (PPV) as an active electroluminescent layer. Devices using Al as a cathode had low brightness and low efficiency and later on D. D. C. Bradley, Synth. Metals 54 (1993)401 had shown that these devices operate at very high voltages (˜40 volts). In a research paper in Advanced Materials 4(1992) 36, the problem associated with the generation of blue light by a LED is discussed and a blue light emitting PPP-LED is proposed which includes the ITO substrate and the aluminum electrodes. The application of double stranded poly-p-phenylene LPPP with partially protonized side groups in electro luminescence devices was first reported by G. Grem and G. Leising in Synthetic Metals (1993). Oligo phenylenes have also been successfully employed before as light emitting layers in electro luminescent devices as discussed by W. Graupner et al. in Mol. Cryst. Liq. Crystal, 256 (1994) 549. However, the conjugation length was at most equal to the length of the hexa phenylenes, with most of light emission in UV-region which is not only invisible to human eye but is also hazardous.
U.S. Pat. No. 6,353,082 relates to preparation of fluorene copolymers comprising disubstituted fluorine moieties and two other moieties containing delocalized pi-electrons which leads to making of a polymer light emitting diodes. These copolymers have excellent solubility in common organic solvents. However, the preparation involves first the synthesis of 9,9′-disubstituted 2,7-dibromofluorene at −78° C. under nitrogen. The process involves multistage synthesis and the resultant copolymer gives a red LED device.
U.S. Pat. No. 6,117,529 provides an electroluminescent device made using double stranded poly-p-phenylene (LPPP) with alkylated side chains attached to the matrix. The process involves multi steps for the preparation of polymer and moreover involves the incorporation of Coumarin 102 dye in the polymethylmethacrylate matrix.
U.S. Pat. No. 6,404,126 describes an electroluminescent device containing a light emitting layer containing a conjugated polymer poly phenylenevinylene (PPV) having substituent attached on the phenylene ring, an inorganic insulative electron injecting and transporting layer consisting of lithium oxide, rubidium oxide, potassium oxide etc. However, the patent relates to the development of device using PPV, commercially available and a costly precursor polymer.
U.S. Pat. No. 5,965,281 involves the use of substituted polyphenylene vinylene derivative, MEH-PPV (Poly[2methoxy-5(2′-ethylhexyloxyl)-1,4phenylenevinylene) as the polymer for the fabrication of device by incorporating lithium nonylphenoxy ether sulphate as the surfactant additive. This has resulted in the fabrication of LED device with better efficiency, but involves the use of costly monomers and precursor polymers.
U.S. Pat. No. 5,241,044 relates to a process for the preparation of soluble poly poly-p-phenylenes by reacting dihydroxy aromatic compounds such as orthoquinone and bisphenols with halogen substituted aliphatic sulphonic acids, halogen sulphonic acids or their anhydrides. The process involves the preparation of the polymer by using zero valence Ni catalyst. By this process poly-phenylenes have been synthesized but its electroluminescence has not been studied.